1. Field of the Invention
The present invention generally relates to the art of microelectronic integrated circuits, and more specifically to a microelectronic integrated circuit mounted on a circuit board with a solder column grid array interconnection, and a method of fabricating the column grid array.
2. Description of the Related Art
Packaged integrated circuits are advantageously mounted on printed circuit boards or motherboards using surface mount technology. A preferred form of this technology is known as Ball Grid Array (BGA) mounting, and includes forming conjugate planar arrays of mounting pads or contacts on surfaces of integrated circuits and circuit boards, and interconnecting the contacts using arrays of solder balls.
As viewed in FIG. 1, a conventional microelectronic assembly 10 comprises a packaged integrated circuit 12 which is mounted on a printed circuit board 14 by means of an array of electrically conductive solder balls 16. The contacts on the circuit 12 and board 14 are not explicitly shown for simplicity of illustration.
The solder balls 16 are made of 90 Pb--10 Sn alloy having a relatively high melting point in excess of 300.degree. C. The balls 16 are adhered to and electrically interconnected with the contacts on the integrated circuit 12 by fillets 18a of eutectic Pb--Sn solder having a relatively low melting point of approximately 220.degree. C. The balls 16 are similarly adhered to and electrically interconnected with the contacts on the circuit board 14 by fillets 18b of eutectic Pb--Sn solder
The balls 16 are conventionally attached to the integrated circuit 12 and circuit board 14 using a fixture or boat made of graphite or similar material that is drilled with holes in a pattern matching the desired solder ball array. The 90 Pb--10 Sn solder balls 16 are loaded into the holes in the boat, and eutectic solder paste is screened directly onto the array. The integrated circuit 12 is aligned on top of the ball array, and heated in a furnace at a profile that melts only the eutectic solder to form the fillets 18a and 18b.
The integrated circuit 12 can have a variety of configurations. As illustrated in FIG. 1, the circuit 12 comprises a ceramic substrate 20 including one or more electrically insulative layers and electrical metallization layers. An integrated circuit chip 22 is attached to the top of the substrate 20, and electrically interconnected therewith by wire bonds 24 or the like. The chip 22 and wire bonds 24 are environmentally protected from physical damage and contamination by an encapsulation 26 formed of epoxy resin or the like.
Another conventional ball grid array mounting configuration is illustrated in FIG. 2, in which like elements are designated by the same reference numerals used in FIG. 1. A microelectronic assembly 30 comprises an integrated circuit 32 which is mounted on the circuit board 14 in the same manner as the integrated circuit 20. However, the circuit 32 comprises a substrate 34 having layers 34a, 34b and 34c. The layers 34a and 34b are formed with openings that define a cavity 36.
The integrated circuit 22 is mounted on top of the layer 34c in the cavity 36, and interconnected with the substrate 34 by the wire bonds 24. The cavity 36 faces away from the circuit board 14, whereby the configuration of FIG. 2 is known as "cavity-up" mounting. The cavity 36 provides ease of wire bonding and improved cooling, and is sealed by an encapsulation 38.
Another conventional assembly 40 is illustrated in FIG. 3, and is known as "cavity-down" mounting. In this case, an integrated circuit 42 includes a substrate 44 having layers 44a and 44b, with an opening being formed in the layer 44b to constitute a cavity 46 which faces the circuit board 14. The downwardly facing cavity 46 is protected with an encapsulation 48.
Several problems are associated with ball grid array configurations. One problem is that, due to the relatively small height and approximately unity aspect ratio of the solder balls 16, the arrays are susceptible to mechanical and thermal stresses which can result in an unacceptably high failure rate.
Another problem is that, especially in the cavity-down configuration of FIG. 3, a spacing "s" between the encapsulation 48 and the facing surface of the circuit board 14 is quite small. Since the balls 16 are mounted using flux solder, residual flux must be removed by a cleaning operation prior to encapsulation. The small spacing s makes the cleaning difficult. In addition, the tolerances for the encapsulation 48 must be maintained precisely to prevent the encapsulation 48 from being too thin or from contacting the circuit board 14.
These problems are alleviated by means of a column grid array (CGA) mounting configuration as illustrated in FIG. 4. In an assembly 50, the cavity-down integrated circuit 42 and circuit board 14 are the same as in FIG. 3. However, the ball grid array is replaced with a column grid array including cylindrical solder columns 52 which are formed of 90 Pb--10 Sn solder and attached using fillets 54a and 54b of eutectic Pb--Sn solder.
The columns 52 provide a larger spacing between the integrated circuit 42 and the circuit board 14, and thereby a larger spacing "s'" between the encapsulation 48 and the board 14. Whereas the solder balls 16 typically have a diameter of 30 mils (0.762 mm), the solder columns 52 typically have a height of 50 mils (1.27 mm) and diameter of 20 mils (0.508 mm). The increased height and aspect ratio of the columns 52 provide greater stress relief, ease of flux cleaning, and relaxed encapsulation height tolerance than the balls 16.
The column grid array assembly 50 is fabricated in a manner similar to the ball grid array assemblies, with the columns being inserted into drilled holes in a graphite fixture or boat and then attached to the integrated circuit 42 and circuit board 14 by the eutectic fillets 54a and 54b.
Solder balls can be fit into holes by pouring an excess number of balls over the surface of the boat, and shaking the boat to remove balls that did not fall into holes. However, the columns 52 are more difficult to insert into holes in a graphite boat than the balls 16. Due to the elongated shape, the columns 52 must be individually inserted into the holes in the boat. The small size of the columns, and the large number of columns (typically several hundred) in a typical column grid array, make this operation difficult and time consuming.